This project focuses on a number of structural determinants responsible for regulating the assembly properties of the actin cytoskeleton and the in vivo consequences of these activities. One of the principle features of actin filament assembly that is not currently understood is the mechanism by which ATP hydrolysis effects actin polymerization. At X9B, we have now obtained high resolution data of Mg-ATP yeast actin and both the Mg-ATP and Mg-ADP forms of Dictyostelium actin. The yeast structure refined at 1.9E clearly shows the adenine nucleotide and the octahedral coordination of the magnesium ion, as well as a water molecule positioned to act as the attacking nucleophile. Furthermore, an extensive network of water molecules is revealed in the large cleft of the actin monomer that might be important in modulating the conformational rearrangements that occur upon filament elongation. These structures will for the first time allow for actin catalyzed ATP hydrolysis to be treated a true enzymatic reaction.